Thermoferrite switch with ptc resistor temperature compensation

ABSTRACT

Disclosed are temperature responsive switches wherein a permanent magnet cooperates with a piece of thermoferrite (TF) to close or open contacts depending on whether the temperature of the TF is below or above its Curie point and hence is or is not attracted by the magnet. A positive temperature coefficient (PTC) resistor is connected electrically in series with the TF and is in intimate heat conducting relationship with it. The TF and PTC resistor are heated by passing current through them and the PTC resistor limits the ultimate temperature attained by greatly increasing its resistance as a predetermined upper limit temperature is attained.

Qarstm 14 1 Sept. '18, 1973 [5 THERMOFERRITE SWITCH WITH PTC 2,718,569 9/1955 Johnston 335/208 RESISTO TEMP 3,057,978 10/1962 Huetten, 335/208 COMPENSATION h n m Primary Examiner-Harold Broome [75] Inventor: Kenneth G. Carson, New Market, Attorney Lamom Koontz et Ontario, Canada 731' Agignee'; arise warm",mifinaapaia'nififij [57] ABSTRACT ,7 M Disclosed are temperature responsive switches wherein [22] Filed: May 11, 1972 a ermanent' ma net coo crates with a piece of therf (TF) 8 1 p d d' mo errlte to c ose or open contacts epen ng on [21] Appl' 252173 whether the temperature of the TF is below or above its Curie point and hence is or is not attracted by the [30] Foreign Application Priority Data magnet. A positive temperature coefficient (PTC) re- Aug. 6, 1971 Canada 120024 Sister is connected electrically in Series with the TF and 1 is in intimate heatconducting relationship with it. The 521 11s. on 335/146, 219/495, 335/208 and resistor heated y Passing current 511 int. c1. 110111 61/013 through them and the PTC resistor limits ru e [58] Field o1Search..'..; 335/146, 208; teiiipel'illili'e attained y greatly lilci'easlng'llls'l'e'slse 219/495, 504, 505; 317/133, 41 318/221 tance as a predetermined upper limit temperature is attained. [56] References Cited UNITED STATES PATENTS 2/1971 Poulsen 335/146 EIaimsJS Drawing Figures THERMOFERRITE SWITCH WITH PTC RESISTOR TEMPERATURE COMPENSATION This invention relates to temperature responsive switches and particularly to temperature compensated temperature responsive switches.

Temperature responsive switches are known in the art, these using a piece of thermally sensitive magnetic material co-operating with a permanent magnet. For example, the thermally sensitive magnetic material may be attached to a contact carrying arm so that the permanent magnet attracts it and causes closing of a pair of contacts, a stationary contact and a contact carried by the arm. As long as the thermally sensitive magnetic material remains below its Curie temperature, the contacts remain closed. However, if the temperature of the material is raised above its Curie point, it loses its magnetic properties and is no longer attracted by the permanent magnet. Biasing means, such as the springiness of the contact arm, causes the contacts to open.

The thermally sensitive magnetic material may comprise an alloy or a thermoferrite material. In the case of thermoferrite material, however, a problern has been encountered in that it has been discovered to have a negative temperature coefficient of resistance. This negative characteristic has resulted in problems of thermal runaway leading to self-destruction. That is, when the thermoferrite material is heated by passing current through it, its resistance decreases, resulting in an increase in current, a further decreasein resistance, and so on which can destroy the thermoferrite material. Also, problems of instability in the thermoferrite temperature'have been encountered due tovariations in ambient temperatures.

To prevent thermal runaway, it is necessary to provide temperature control of the thermoferrite. Also, to reduce the problem of ambient temperature compensation, a method to control the ambient temperature immediately surrounding the thermoferrite switch area is required. It has been discovered that both of these problems can be overcome with the use of positive temperature coefficient resistors, as will be explained in detail hereinafter.

For brevity, the following disclosure will frequently refer to thermoferrite material simply as TF material and to the positive temperature coefficient resistor as simply a PTC resistor.

As is known, a PTC resistor has a characteristic of increasing its resistance on a temperature rise. Various types are available commercially and a preferred type for the present invention is one in which the resistance change is rapid over a narrowtemperature band, acting almost as a switch.

The PTC resistor may beapplied by, connecting it in series with the thermoferrite chip, attaching the PTC resistor to the TF material via heat conductive, electrically conductive epoxy adhesive. This allows the PTC resistor to read" the thermoferrite temperature.

In operation, when a voltage is applied to the input connections, a current flows through the TF material and the PTC resistor. This causes internal heating of the ,TF material and PTC resistor, and also heat may flow from one to the other so they assume a common temperature. When the PTC resistor attains the temperature of its switching point wherethe resistance increases drastically, it limits the current in the circuit, and it stabilizes at this temperature, thus stabilizing the temperature of the TF material. If the operating temperatures of the TF material and the PTC resistor are selected so that the Curie temperature of the TF material is below the switching temperature of the PTC resistor, then the TF material will be allowed to heat through its Curie temperature, and will be prevented from heating beyond the PTC resistors switching temperature. Thus is eliminated the danger of thermal runaway of the TF material. A

This control effect is preferably available regardless of the ambient temperature surrounding the device, at least within reason, as the PTC resistor will compensate its resistance value to maintain a current flow sufficient to hold the PTC/TF assembly at the PTC resistors switching temperature, until the demand exceeds the input available to the circuit. When this 'point is achieved, rapid decrease in temperature will be noticed and the TF material will go below its Curie temperature.

If desired, the TF material and the PTC resistor may be separate items electrically connected in series and bonded together in intimate heat-conducting. relationship. Alternatively, the two may be combined into a chip of TF material and PTC resistance material between a' pair of terminal plates using bonding material which is both electrically and thermally conductive.

According to one aspect of the invention, there is provided a temperature responsive switch comprising means supporting a pair of electrically conductive contact carrying arms in electrically isolated, spaced apart relationship. One of the arms is flexible so that its contact may move between first andsecond positions into or out of engagement with the contact on the other-arm of the pair. This arm also carries a permanent magnet adapted to co-operate with a fixed piece of thermoferrite material which attracts the permanent magnet when its temperature is below its Curie temperature but does not attract the magnet when its temperature is above the Curie temperature. The piece of thermoferrite material-is electrically connected in series with a positive temperature coefficient resistor between a pair of control terminals and the piece of thermoferrite material is in intimate heat conducting contact with the positive temperature coefficient resistor.

According to another aspect of the invention, there is provided a temperature responsive switch comprising means supporting a pair of electrically conductive contact carrying arms in electrically isolated, spaced apart relationship. One of the arms is flexible so that its contact may move into or out of engagement with the contact on the other arm of said pair. This arm also carries a chip comprising a piece of thermoferrite material electrically connected in series with a'positivetemperature coefficient resistor between a pair of control terminals, the thermoferrite material being in intimate heat conducting contact with the positive temperature coefficient resistor, the contacts on the arms being normally open. The switch further comprises a pushbutton carrying a permanent magnet movable towards the chip when the push-botton is depressed to thereby attract the chip when it hasa temperature below its Curie temperature. The push-button is spring biased outwardly whereby the permanent magnet and chip move outwardly to close the contacts on the arms if the thermoferrite is below its Curie temperature.

The contacts, if closed, may be caused to open fol-- lowing a time delay by applying'current through the chip via the control terminals sufficient to cause the thermoferrite to heat above its Curie point whereby it ceases to be attracted by the permanent magnet.

The invention will now be further described in conjunction with the accompanying drawings, in which:

FIG. 1 is a diagram of one embodiment of the invention.

FIG. 2 comprises graphs illustrating temperature versus resistance of the TF and PTC and also showing the TF Curie temperature.

FIG. 3 shows a modification of the FIG. I embodiinent, 7

FIG. 4 is a detail of the TF/PTC chip used in the FIG. 3 embodiment, and

FIG. 5 shows another embodiment according to the invention.

Referring to FIG. 1, this embodiment is seen to comprise a base plate supporting a switch stack 12 made up of insulating spacers and contact carrying arms 13 having connecting terminals 14 and carrying switch contacts 15 which are normally open, FIG. 1 illustrating the open contact position of the switch. Secured to the lower arm 13 is a permanent magnet 16 adapted to co-opcrate with a piece of thermoferrite material 18 secured to the base plate 10 by, for example, epoxy. Secured to the underside of base plate 10, which ismade of metal so as to be thermally conductive, is a positive temperature coefficient (PTC) resistor 20. The PTC resistor 20 is in intimate heat-conducting relationship with TF material 18.

The spacing between contacts 15, when in the open position, may be adjusted by means of an adjustment screw 21 in support plate 22 secured to switch stack 12, the stack 12 being held together by suitable means such as a bolt 23 and a nut, not shown.

In the position shown in FIG. 1, it has been assumed that the TF material 18 is below its Curie temperature so that it attracts magnet 16 and contacts 15 are open. By applying a control current through the PTC resistor 20 and TF material 18 via terminals 25 and 26, they are caused to heat up and when the TF material heats to its Curie temperature, it releases permanent magnet 16 so that contacts 15 close to complete a circuit to a device connected to output'terminals 14. Further heating of the PTC resistor 20 and TF material by the control current will eventually result in the PTC resistor attaining the temperature of its switching point where its resistance increases drastically, at which time it will limit the current in the circuit and will stabilize at this temperature, thus stabilizing the temperature of the TF material. If the operating temperatures of the TF material and the PTC resistor are selected so that the Curie temperature of the TF material is below the switching temperature of the PTC resistor, then the TF material will be allowed to heat through its Curie temperature, but will be prevented from heating beyond the PTC switching'temperature so that thermal runaway cannot occur.

This control effect is available independently of the normal ambient temperatures surrounding the device, as the PTC resistor will compensate its resistance value to maintain a current flow sufficient to hold the PTC/TF assembly at the PTC resistor switching temperature, until the demand exceeds the input available to the circuit. When this point is achieved, rapid decrease in temperature will be noticed, and the TF material will go below its Curie temperature.

FIG. 2 is largely self-explanatory and needs little explanation. I-Iere resistance versus temperature is shown for both the TF material and the PTC resistor in arbitrary units of resistance and temperature. The TF materials Curie temperature is also indicated and it can readily be seen that the resistance of the PTC resistor rapidly increases in a narrow temperature band higher than the Curie temperature of the TF material. The negative temperature coefficient of resistance of the TF material is also evident from this drawing.

In the FIG. I embodiment, any effect that would cause a temperature change in the assembly is automatically compensated for by the PTC resistor. A decrease in assembly temperature will cause a reduction in resistance of the PTC resistor, allowing a higher current flow which will give higher heating input, driving assembly temperature back to the switching temperature of the PTC resistor. The opposite effect occurs on an increase in assembly temperature. Thus, the PTC resistor automatically regulates the assembly temperature, and compensates for external conditions which affect the. assembly temperature. These external conditions would relate to ambient temperatures surrounding the device, variation in applied control voltage and similar variations. Normally the assembly consists of a PTC resistor selected with a switching temperature above the selected Curie temperature of the TF material so as to provide, in effect, an upper limit control.

The entire switching mechanism shown in FIG. 1 acts as a relay. A current flow change sufficient to shift the temperature of the thermoferrite chip through its Curie point causes switching of contacts 15. From the previous statements, it may be seen that the function of the PTC resistor in the assembly is utilized to stabilize the temperature of the assembly at a predetermined temperature above the Curie point of the thermoferrite regardless of the ambient temperature (within reason) or other variations. A decrease in voltage applied to the TF/PTC assembly can then be used to cause the thermoferrite to cool through its Curie temperature. This characteristic affords a time delay action, with time variation being obtainable by varying the difference between the thermoferrite Curie point and the PTC resistors switching temperature, and/or by variation in input voltage rate of change. The device therefore provides a thermal time delay relay characteristic.

The embodiment shown in FIG. 3 operates in exactly the same manner as the embodiment shown in FIG. 1 but here the TF material and PTC resistor have been combined into a chip" which is shown enlarged in FIG. 4.

Referring to FIG. 4, the chip is seen to comprise a conductive terminal/mounting plate 30 provided with holes 31 through which screws, for example, may be passed to mount the assembly on the base plate 10 (FIG. 1 and FIG. 3). A chip of thermoferrite material 32 is bonded at 33 to the tenninal/mounting plate 30 and at 34 to a piece of PTC resistor material 35. The PTC resistor material 35 is bonded at 36 to a terminal piece 37. The bonding material at 33, 34 and 35 is both electrically conductive and thermally conductive. Electrical connections are made to 30 and 37 so that current flows in series through TF material 32 and PTC resistor 35 and heat is generated in the TF material 32 and PTC resistor 35. Because of the intimate thermal contact between the TF material and PTC resistor, they assume substantially a common temperature.

From FIG. 4 which is a large scale drawing, it might seem that the mounting plate is quite thick but in practice it is thin, e.g. brass shin stock, and of course must be thin because the permanent magnet exerts its attraction through this material. In fact if necessary it could be cut away so that the permanent magnet comes in direct contact with the face of the thermoferrite.

The assembly shown in FIG. 4 can be utilized as a self-limiting heating unit. The assembly, when mounted on a surface, and voltage applied to it, dissipates heat into the surface at a rate controlled by the input voltage/resistance characteristic of the assembly. Selflimiting of temperature is attained when the surface reaches the switching temperature of the PTC resistor. Also, it is possible to build a two-stage device, using an additional P'IC resistor chip in series. This provides a two-temperature selection. One PTC resistor chip with a switching temperature below the TF materials Curie point is selected by a control circuit to stabilize temperature below the TF materials Curie point. If the has a contact mechanism which opens on a temperature rise, an ambient temperature abovethe Curie point of the device will render the contact mechanism open at all times, thus providing upper limit function. At ambient temperatures below the TF materials Curie point, normal relay action will occur. Thus there results a thermal relay capability with automatic upper temperature limit on ambient operation.

FIG. 5 shows an embodiment according to the invention which provides an open contact mode after a time delay period, and has a characteristic preventing contact closure until after a desired cool-down period.

The device comprises an enclosure 50 having a stationary contact 51 on a contact arm 52 which extends through the casing. A movable contact 53 is provided on contact arm 54 which also extends through the'casingA load to be controlled may be connected across the contactcarrying arms 52 and 54.

The arms 54 carrying contact 53 also carries a chip" comprising a piece of TF material and a PTC. resistor electrically in series and in heat conducting relationship similar to the arrangement shown in FIG. 4.'This' chip is connected over wires 60 to contacts 61 extending through the casing and to which control current may be applied.

Assuming the TF material is at a temperature below its Curie point, it will be attracted by the permanent magnet 62 and retained in the closed position shown in FIG. 5. If a control current is passed through the TF/PTC-chip so that thevTF material heats above its Curie point, it will become non-magnetic so that mag net 62 no longer attracts it. The arm 54, being of springy material, will move. the chip downwards along with contact 53, i.e. contacts 5 and 53 will become separated to open the circuit to the load.

If the reset button is immediately pushed, i.e. before the temperature of the TF material goes below its Curie point, the magnet 62 will be moved down into contact with arm 54 to which the TF/PTC chip is secured but will not pull this up when the reset button is released because the TF material is still non-magnetic. It will be appreciated that the TF material is adjacent the arm 54 on its under side so as to be closer to the permanent magnet 62 than the PTC resistor.

The reset button to which the magnet 62 is secured is mounted on a leaf spring 64 secured to the casing by suitable means such as a rivet 65. Thus the reset button is biased outwardly by spring 64 but may be manually pushed inwardly.

If the reset button is pushed after the TF material has had time to cool below its Curie point, magnet 62 will be brought sufficiently close to the TF material, to attract it to itself and when the .reset button is released, it will pull up the TF/PTC chip along with the free end of the contact arm 54 carrying contact 53 and hence contact 53 will engage stationary contact 51 to again close the circuit to the load.

While the above embodiments are single-pole singlethrow devices it will be obvious to those skilled in the art that multiple-pole, multiple-throw devices could also be constructed along similar lines, i.e. with FTC resistor compensation. I

The embodiments of FIGS. 1- and 3 have normallyopen contacts; i.e. when the TF materialis below its Curie temperature, it attracts magnet 16 and contacts 15 are open. Obviously it would be simple to provide normally-closed contacts. For example a contact could beprovided on the. underside of the movable contact carrying arm for cooperation with a stationary contact undemeaththe movable arm. A flasher type of circuit can be made by connecting such normally-closed contracts in series with the TF material and PTCresistor and a current source. When the current heats the TF materialto its Curie point the contacts open; when the TF material cools, the contacts close agaimand so on. I

InFIG. 5 it appears that there is no electrical isola-. tion between the lower load contact arm and the TF- PTC chip and hence between the load circuit and heater circuit. However it should be noted that these circuits can be completely isolated by ensuring that the TF-PTC chip is insulated from the lower load contact by an insulating epoxy or by an alternative mechanica mounting arrangement.

The Curie temperature of the thermoferrite material should be selected to be above any ambient tempera-' tures expected to be encountered by the devices ac-' cording to the invention. Otherwise, the thermoferrite material might never cool below its Curie point and'the devices wouldnot function properly.

THE EMBODIMENTS OF .THE INVENTION IN WHICH AN EXCLUSIVE PROPERTY OR PRIVI- LEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

said magnetic means for supplying heat thereto,

whereby said magnetic means, dependent upon its magnetic state, attracts or releases said permanent magnet means to operate said'contact means.

2. A temperature responsive switch of claim 1 wherein said thermally responsive magnetic means comprises a Curie point temperature responsive thermoferrite which is magnetic when the temperature to which said thermoferrite is exposed is below said Curie point temperature and wherein said thermoferrite is non-magnetic when the temperature to which it is exposed is above said Curie pointtemperature.

3. The temperature responsive switch of I claim 2 wherein said contact means comprises a normally open switch having first and second switch arms and wherein said permanent magnet means is fixed to one of said arms.

4. The temperature responsive switch of claim 1 wherein said contact means comprises a normally closed switch having a first switch arm and a second flexible switch arm and further comprising a spring bias reset button having said permanent magnet means affixed thereto for resetting said normally closed switch. 5. The temperature responsive switch of claim 4 wherein said thermally responsive magnetic means comprises a thermoferrite having a Curie point temperature below which said thermoferrite is magnetic and above which said thermoferrite is non-magnetic.

6. The temperature responsive switch of claim 5 wherein said thermoferrite and said PTC resistor means are so constructed to form a chip attached to said flexible switch arm. 

1. A temperature responsive switch comprising: circuit controlling contact means; permanent magnet means attached to said contact means; thermally responsive magnetic means having states of magnetism or non-magnetism dependent upon temperature, said magnetic means being positioned in controlling relation to said permanent magnet means; and PTC resistor means connected in electrical series to said magnetic means for supplying heat thereto, whereby said magnetic means, dependent upon its magnetic state, attracts or releases said permanent magnet means to operate said contact means.
 2. A temperature responsive switch of claim 1 wherein said thermally responsive magnetic means comprises a Curie point temperature responsive thermoferrite which is magnetic when the temperature to which said thermoferrite is exposed is below said Curie point temperature and wherein said thermoferrite is non-magnetic when the temperature to which it is exposed is above said Curie point temperature.
 3. The temperature responsive switch of claim 2 wherein said contact means comprises a normally open switch having first and second switch arms and wherein said permanent magnet means is fixed to one of said arms.
 4. The temperature responsive switch of claim 1 wherein said contact means comprises a normally closed switch having a first switch arm and a second flexible switch arm and further comprising a spring bias reset button having said permanent magnet means affixed thereto for resetting said normally closed switch.
 5. The temperature responsive switch of claim 4 wherein said thermally responsive magnetic means comprises a thermoferrite having a Curie point temperature below which said thermoferrite is magnetic and above which said thermoferrite is non-magnetic.
 6. The temperature responsive switch of claim 5 wherein said thermoferrite and said PTC resistor means are so constructed to form a chip attached to said flexible switch arm. 